A. Field of the Invention
The present invention relates to a torque converter, and in particular to a torque converter having a lockup clutch.
B. Description of the Background Art
Generally, a torque converter provides an automotive vehicle with a smooth acceleration and deceleration since torque transmission is carried out via movement of a working fluid or oil. However, energy loss is experienced due to the general nature and physical characteristics of the working fluid, so that fuel efficiency of an automotive vehicle with a torque converter is not necessarily optimal.
Some conventional torque converters employ lockup mechanisms for mechanically connecting a front cover of the torque converter to the turbine or output of the torque converter. The lockup mechanism is disposed within a space defined between the turbine and the front cover. Typically, the lockup mechanism includes a disc-shaped piston member which can be urged into contact into with the front cover and a drive plate attached to a backside or other portion of the turbine. Usually coil springs are incorporated into the lockup clutch mechanism for elastically coupling the piston member and the driven plate in the circumferential direction to absorb vibrations and allow limited relative rotary displacement between the driven plate and the piston member. An annular friction element is formed on one portion of the piston member at a position opposed a corresponding planar friction surface of the front cover.
In the conventional lockup clutch described above, the working operation of the piston member is controlled by manipulation of the flow of working oil in the torque converter. Specifically, the direction of the flow of the working oil within the torque convertor determines whether the piston member and the front cover are engaged with one another or not. With the lockup mechanism in the engaged state, working oil in the space between the piston member and the front cover may be drained from a radial inner portion of the torque convertor and the piston is pressed into contact with the front cover. As a result, torque is directly transmitted from the front cover to the turbine via the piston member and the coil spring.
In the conventional lockup mechanism, as described above, the piston member is controlled by working oil flowing inside the torque converter. Therefore, the engagement and disengagement operations cannot always be precisely carried out due to the turbulent flow of the working oil within the torque convertor. Moreover, when the lockup clutch engagement and disengagement operations are carried out, the response time is generally slow. Specifically, the amount of time that passes beginning with the instant lockup clutch engagement is determined necessary until the lockup clutch actually engages is considerable. Further, in the conventional lockup mechanism, working oil cannot flow the portion between the friction surface of the front cover and the wet friction facing on the piston member, so that the wet friction facing might not be sufficiently cooled by adjacent flow of fluid, thereby shortening the service life of the wet friction facing.
Conventionally, sufficient amounts of torque are not always transmitted in the lockup mechanism when a single friction surface is used. Therefore, a multiple disc lockup clutch having a plurality of friction surfaces formed on a plurality of friction plates is often used in torque convertors. In such a multiple disc lockup clutch, however, each friction plate must be displaced in order for the lockup clutch to move from the engaged state to the disengaged state, in order to reduce drag forces that are often generated.